Freshly demoted Ensign Tom Paris is placed in Voyager 's brig for thirty days for disobeying orders. During this, framing the episode, Paris relates the events to his personal log as a message to his father.
In flashback, Voyager is shown meeting with the non-native delegates of an ocean planet. The delegates, including Riga and Burkus, explain that the planet is losing water mass at an alarming rate, threatening to destroy the planet in 5 years if it is not stopped. Though they have some submarine vehicles, they are unable to reach the center of the planet where they believe the source of their problems can be found. The Voyager crew offers the use of the Delta Flyer, capable of withstanding the pressures at the planet's center. Paris, piloting the excursion with some of the planet's scientists including Riga, discover that the planet's core is a massive reactor activated over a hundred thousand years ago, drawing the surface water of a nearby planet to it for some unknown reason. Further exploration reveals that oxygen-mining reactors used by the current population is the cause of the water loss.
The Voyager crew offer several technology options to reduce and minimize the water loss and other more efficient technologies, but Burkus seems impassive about them. Even when Riga suggests turning off several of the oxygen-mining units, significantly prolonging the planet's existence but potentially impacting the quality of life for the inhabitants, Burkus simply offers to pass these ideas to their government. The crew is eventually told that the government will not turn off the oxygen-mining facilities but politely thanks for them for their assistance.
Urged on by Riga, Paris attempts to convince Captain Janeway to change the minds of the inhabitants, but Janeway refuses, citing that Voyager cannot get involved due to the Prime Directive. Furious at the decision, Paris and Riga steal the Delta Flyer, and attempt to use a photon torpedo to destroy one of the oxygen-mining facilities, but the remaining crew destroy the torpedo with a depth charge and recover the Delta Flyer. On return to Voyager, Janeway demotes Paris to the rank of ensign for disobeying orders and orders him to the brig for thirty days.
After the flashback ends, Tuvok goes to the brig and releases Paris, saying that his thirty days have been served and tells him to report for duty and also tells him to clean himself up as Paris leaves the brig with Tuvok. Paris is in his quarters after cleaning himself up and ends the message as he sends it to his father.
The original shoot of this episode was wholly the scenes of Tom Paris' adventure on the planet. When test screened the episode lacked the mandatory time length and so the brig scenes of the episode were formulated by Robert Beltran (Commander Chakotay) and added.
The Monean scientist's instrument shown in the episode "Thirty Days" being the round prop carried by Riga for the purpose of measuring depth, pressure, etc., is the same prop described in the later episode "Dark Frontier" where it is described by Harry as a Borg "autonomous regeneration drone." and by Janeway as some sort of polytrinic alloy.